Abstract: An inverting and reversing prism system for a stereo microscope having an optical longitudinal axis and left and right eye viewing systems having left and right longitudinal axes, respectively, along which left and right beam paths are transmitted, respectively, comprising: at least four reflecting surfaces (13, 15, 16, 13A and 13A, 15A, 16A, 13) in each beam path (14, 14A), two of the reflecting surfaces (13, 13A and 13A, 13) in each beam path comprising inlet and outlet surfaces, respectively, for deflecting the beam path by 90.degree., the inlet and outlet surfaces in each beam path being oriented 45.degree. relative to the optical longitudinal axis of the microscope and 90.degree. relative to each other, with a line of intersection (26) of planes containing the inlet and outlet surfaces in each beam path defining an angle of 45.degree.